NEW YORK (Reuters Health) - The heavier you are, the greater your odds of getting the painful arthritic condition gout - no matter your gender or race, a new U.S. study finds.

Obesity has long been considered a risk factor for gout. But the new findings confirm that the risk starts climbing when people are merely overweight - and that more Americans are developing the condition these days.

Gout is a particularly painful form of arthritis that causes the joints to periodically become swollen, red and hot - most often affecting the big toe, though it also strikes the feet, ankles, knees, hands and wrists.

Historically, research into gout has mainly used data on white men. The new study, reported in the journal Arthritis Care & Research, looked at a broader swath of Americans.

"Gout is not just a disorder of men. It's not just a disorder of whites," said senior researcher Dr. Allan C. Gelber, of Johns Hopkins University School of Medicine in Baltimore.

Using data from a periodic government health survey, Gelber's team found that between 1988 and 1994, about 2.6 percent of U.S. adults had gout - corresponding to 4.7 million people.

Between 2007 and 2010, that figure was almost 3.8 percent, or about 8.1 million adults.

Regardless of the time period, though, obese Americans had the highest gout rates.

In the later years, almost 5.5 percent of moderately obese adults had gout, as did seven percent of severely obese people.

That compared with 1.6 percent of normal-weight adults and 3.4 percent of overweight people.

"The increase in gout risk applies to people who are overweight as well," Gelber said. "It's not that there's a threshold where it goes up only if you're obese."

In addition, extra pounds seem to be an equal-opportunity risk factor. Women were less likely than men to have gout at either time period, but excess weight was linked to a similar impact on their risk.

Weight also seemed to carry as much importance for African Americans and Mexican Americans as it did for whites. There weren't enough data to single out other ethnic groups.

As for why weight affects gout risk, there are a couple reasons, but questions remain.

Gout arises when uric acid crystals build up in the joints. The body produces uric acid when it breaks down purines - substances found naturally in the body, but also in certain foods, like organ meats, anchovies, mushrooms and some seafood, such as herring and mackerel.

Obesity, as well as high blood pressure, diabetes and kidney disease can boost the body's production of uric acid or slow the clearance of it.

But based on the current findings, uric acid and other health conditions do not fully explain why overweight people have a higher gout risk. (The study participants - more than 28,000 in all - underwent physical exams and blood tests, including uric acid measurements.)

"They're part of the story, but not the whole story," Gelber said. Researchers are still trying to figure out the other reasons for the weight-gout connection, he noted.

According to Gelber, people should be aware that "even small elevations in weight are associated with a modest escalation in gout risk."

He and his colleagues estimate that for someone who is 5 feet, 9 inches tall, every seven pounds of extra weight corresponds to a five-percent increase in gout risk.

Gelber said gout should be on doctors' minds when an overweight patient complains of joint pain. That's important, he said, because there is a difference in the treatments for gout and osteoarthritis - the common "wear-and-tear" form of arthritis.

Both can be managed with anti-inflammatory pain medications. But with gout, a doctor might also prescribe medications that lower uric acid levels, or make other recommendations - like limiting alcohol and purine-rich foods.

Gout is usually diagnosed based on symptoms, uric acid levels in the blood and the presence of crystals in a person's joint fluid.